The present invention relates to solar modules having low weight which have high stability due to the special configuration of its front and back panes. Preferable panes are glass panes and in the following the term “pane” also includes a glass pane.
Normally, solar modules have a design according to FIG. 1. Thus, solar modules normally comprise a front pane 2, a back pane 3 and an intermediate layer 4. As shown in FIG. 2, the intermediate layer often consists of embedment material layers 5 and 6 as well as solar cells 7. The embedment material layers 5 and 6 themselves can be laminates so that they consist of several single layers.
Basically, the solar modules according to the present invention have a similar design. But they are characterized by a special configuration of the front pane and the back pane.
With the special configuration of the front pane and the back pane according to the present invention solar modules are provided which have improved stability and reduced weight in comparison to prior art solar modules. In this way it is possible to provide larger modules having nevertheless a weight which can be handled.
Solar cells for direct conversion of solar energy into electric current have to be encapsulated for mechanical protection e.g. against hail, damages during the installation or service and corrosive environmental influences as well as for achieving the required electrical safety. The essential components of the encapsulation are an optically transparent front pane, embedment materials into which the solar cells are laminated or cast and which are transparent at least between the front pane and the solar cells, and a back foil on the back side of the solar module. The composite consisting of a front pane, embedment materials, solar cells, integrated components, a back foil and a back pane as well as optionally a frame is referred to as a solar module.
Front panes normally consist of natron silicate glasses having a thickness of greater than 3 mm so that they can be thermally pre-tensioned. In rolled front panes structures can be rolled in or embossed on the outer side surface to prevent vectored reflection of light or to increase the coupling-in efficiency for the solar radiation. It is possible to coat the outer surface for improving the coupling in of radiation.
As embedment materials polymer materials, such as e.g. EVA (ethylene vinyl acetate) which is relatively inexpensive and has well known properties, can be used. Normally, the embedment materials are used in the form of foils having thicknesses of between 0.4 and 0.8 mm.
The solar cells can be panels of polycrystalline silicon having thicknesses of between 0.1 and 0.4 mm. Solar cells can also be directly applied onto the front pane. This is normally realized with a transparent and electrically conductive intermediate layer. In some embodiments of thin layer modules solar cells are embedded in the module on separate carrier panels e.g. of electrically conductive metal sheets. In the mechanical realization of the solar modules then the carrier panel with the solar cells replaces the real solar cells.
Many back foils are known, such as TPT® (TEDLAR®-PET-TEDLAR®, TEDLAR® polyvinyl fluoride) and TAP® laminates (TEDLAR® aluminium PET) having normal thicknesses of 0.3 mm to 0.8 mm. For the back side also metal plates or glass panes can be used. The latter are referred to as double glass modules. For that also lime natron silicate glasses are used which normally have thicknesses of higher than 3 mm so that they can be thermally pre-tensioned. Normally, double glass modules have a symmetric design, i.e. the thicknesses of the front pane and the back pane are similar.
At least one of both panes (front pane or back pane) has the function of a carrier. In the case of double glass modules both panes have a carrier function. In this case, both panes absorb the loads, which are caused by their own weight, and by wind, ice and snow, and they transmit them through the frame, clamps at the edges or the like to the substructure of the solar modules.
Solar modules for terrestrial use have to be designed for a temperature range of −40 to 85° C. Since solar modules are normally assembled by laminating at temperatures of up to 150° C., the temperature range in which a solar module has to “survive” is −40 to 150° C. In this case, the load is caused by different coefficients of thermal expansion a of module components. As reference values the following values can be used.                Natron silicate glasses which are normally used for front and back panes have a values in the range of 8*10−6 to 10*10−6 K−1.        Back foils of polymeric materials (e.g. TPT®) have α values in the range of 50*10−6 to 150*10−6 K−1, wherein these values strongly depend on the temperature in a non-linear manner and may change in the course of time.        In the case of back foils of polymeric metal composites (e.g. TAP®) the metal layer has a strong influence on the coefficient of thermal expansion of the whole composite due to its relatively high modulus of elasticity. TAP® has an α value of about 13*10−6 K−1 to 18*10−6 K−1.        For embedment materials of polymers the α values are in the range of 50*10−6 to 200*10−6 K−1, wherein these values depend on the temperature in a non-linear manner and may change in the course of time.        The layer with the solar cells, e.g. of crystalline Si panels has an average coefficient of expansion of about 4*10−6 K−1.        The thermal expansion of cells for thin layer modules in which the cells are directly applied onto the front or back pane can be neglected. If the cells are applied onto separate carrier panels, the mean coefficient of expansion of the carrier panels is used. The “mean coefficient of expansion” means the coefficient of expansion which is the result of the determination of the average of the coefficients of expansion of the materials in the plane of the solar module in consideration of their relative proportions of length.        
In the case of temperature changes mechanical stress in the solar module can be caused by different coefficients of expansion of the solar module components and the solar modules may be deformed. A strong deformation may e.g. lead to high mechanical loads in prefabricated building members and roof structures.
In the past many attempts have been made to provide solar modules with constant stability but with reduced weight. E.g. EP 1 798 775 A2 describes solar modules with front and/or back panes of borosilicate glass. In the relevant temperature range common borosilicate glasses have coefficients of thermal expansion of 3.3 to 6*10−6 K−1 and thus are better adjusted to the coefficient of expansion of the silicon solar cells (ca. 2.6*10−6 K−1) than natron silicate glasses. This may reduce thermo-mechanical stress in the solar modules but does not reduce the weight of the solar modules themselves.
In the case of double glass modules it is known that thermo-mechanical loads can be reduced by a symmetric design: front and back panes are of the same glass (the same coefficient of expansion) and have nearly the same thicknesses. This prevents the “bimetal effect” during temperature changes with strong warping of the panes which leads to high bending stress in the module components and to high mechanical loads in the prefabricated building members or roof structures.
In the case of back foil modules normally thermo-mechanical loads are compensated by the use of viscoelastic or viscoplastic creep ability which is inherent to the embedment materials and the back foil. If the temperature changes are not too fast, a relaxation of the mechanical stress in the embedment materials and foils takes place so that the thermal elongation and shrinkage of these materials only leads to low mechanical stress.
In the case of back foil modules with metal interlayers in the back foils (e.g. TAP®) normally the metal layers are thin enough that the thermo-mechanical elongations do not result in appreciable forces exerted onto the solar module. In the case of strong temperature changes back foils may form convolutions with high creep ability of the embedment materials which are below the foils. But normally they do not result in appreciable mechanical loads of the solar modules.
Typical dimensions of solar modules are 1.1*1.3 m2. Larger modules are preferable, because they have lower production costs, storage costs, transport costs, fewer necessary prefabricated building members and necessary electric circuits. Modules having dimensions of 2.2*2.6 m2 are already under way. Large modules whose weight has not been reduced will cause the following problems.                The handling of large modules is laborious due to their bulkiness and high weight. Normally, solar modules are installed manually. Since the bulkiness is a result of the weight of solar modules, there exists the desire for reducing weight so that also large modules can be carried by one or two persons.        A high weight of single modules makes great demands on the prefabricated building members and roof structures.        Large modules have to tolerate a higher load by their own weight.        Large modules have to tolerate a higher load due to wind, hail and snow loads.        
The solar modules are deformed by wind and snow loads in an elastic or plastic manner or mechanical stress is caused in the module components in particular in the faces, edges and optional passages in the front and back panes.
At best the solar module has elastic or viscoelastic properties. To assess the bending of a solar module caused by wind, snow and its own weight, the module can approximately be considered as a Kirchhoff plate. For a Kirchhoff plate onto which a homogenous pressure is applied to the whole face (test conditions according to standard EN 61215) the bending is defined asW∝L4/t3,wherein w is the rising height of the bending (e.g. the distance from the highest point on the upper side of the front pane to the lowest point), L is a representative length of an edge of the solar module or its diagonal or the diameter in the case of circular modules and t is the thickness of the module. When the maximum bending of the modules is limited, the necessary thickness terf and the weight G of the solar module are defined with respect to the module size:terf∝L1.33 G∝L3.33 I.e., the necessary thickness of a solar module and its weight increase disproportionately to its lateral dimensions. An assessment for that: in the case of a solar module having dimensions of 1.1×1.3 m2 a glass pane having a thickness of 3 mm can be used for the front pane, but for a module having dimensions of 2.2×2.6 m2 a thickness of the glass pane of at least 7.5 mm would be necessary to limit its bending. In this case the weight of the module would increase by a factor of 10. The handling of modules with such a high weight would be very difficult.
The unexamined and first publication DE 10 2005 057 468 A1 describes a solar module which is adhered to a light-weight structure strengthened by support frames for reinforcement. This proposal has several disadvantages: With temperature changes in the adhesion areas between the solar modules and the light-weight structure high shear and normal stress are caused which in the course of time may lead to a breakdown of the composite. The light-weight structure is bulky and weighty. In addition, it requires special substructures.
The unexamined and first publication DE 10 2005 030 039 A1 describes a substructure for fixing solar modules on flat roofs. In this case the forces exerted onto the roofs by wind loads at the modules are reduced by wind baffle plates and wind deflectors. But this solution cannot be used in the case of snow loads. In addition, there is the object to provide large-scale solar modules which can withstand high wind loads.
There is a high demand for large solar modules having low weight.